This invention relates to a laser beam machining apparatus, and more particularly to a machining head for controlling the focal position of the laser beam.
FIG. 2 shows an external appearance of an ordinary laser beam machining apparatus.
In FIG. 2, reference numeral 1 designates a laser beam machining head; 2, a Z-axis unit for moving the machining head 1 vertically (in a direction of Z-axis); 3, a Y-axis unit for moving the Z-axis unit 2 back and forth (in a direction of Y-axis); 4, a column supporting the Y-axis unit 3; 5, a bed; 6, a table moving right and left (in a direction of X-axis) on the bed 5; and 7, a metal cover for protecting a table drive mechanism adapted to drive the table.
FIG. 3 shows the arrangement of the machining head in the conventional laser beam machining apparatus which has been disclosed, for instance, by Unexamined Published Japanese Patent Application No. 159285/1982 or Unexamined Published Utility Model Application 189492/1988.
In FIG. 3, reference numeral 8 designates a laser beam; 9, an optical system, such as a lens, for concentrating the laser beam 8; 10a, an inner holder holding the optical system 9; 10b, an outer holder holding the inner holder 10a; 11, a drive unit for driving the inner holder 10a inside the outer holder 10b in such a manner that the inner holder 10a is moved along the optical axis (vertically in FIG. 3); 12, a nozzle whose outer wall is threaded (not shown) so that it is threadably engaged with the outer holder 10b; that is, the nozzle is turned so as to go in and out of the outer holder to adjust the length of protrusion of the nozzle 12; 13, a workpiece; 14 and 15, an electric motor and a screw mechanism, respectively, which are used for moving the machining head 1; 16, a control unit for applying drive signals to the motor 14; 17, a profiling unit for measuring the distance between the outer holder 10b and the workpiece 13; 18, a signal line for transmitting the output signal of the profiling unit 17 to the control unit 16; and 19, a signal line for applying the output signal of the control unit 16 to the motor 14.
The operation of the laser beam machining apparatus thus organized will be described.
In general, a laser beam machining operation is carried out by applying the laser beam 8, concentrated by the optical system 9, to the workpiece 13. During the machining, it is essential to hold the focal position of the laser beam 8 at a predetermined position with respect to the workpiece 13. For this purpose, the profiling unit 17 is used. The output signal of the profiling unit 17 is applied to the control unit 16, which applies, in turn, a drive signal to the motor 14 so that the distance between the outer holder 10b and the workpiece 13 becomes a predetermined value. Accordingly, as long as the position of the optical system 9 is maintained unchanged inside the outer holder 10b, the focal position of the laser beam 8 is held at a predetermined distance from the upper surface of the workpiece 13.
In the case where, for instance, the laser beam 8 is focused on the upper surface of the workpiece 13 as shown in FIG. 3, the focal position of the laser beam 8 is as follows: That is, as long as the position of the optical system 9 and the settings of other components are maintained unchanged, the focal position of the laser beam 8 is maintained on the upper surface of the workpiece 13 even if the thickness of the workpiece 13 changes.
In a laser beam machining operation, it is also essential to keep the distance (gap) between the nozzle 12 and the workpiece 13 constant, because the gap affects the stream of the machining gas (generally, Oxygen) which is jetted from the nozzle 12 to accelerate the machining operation, thus affecting the machining operation. Generally, the gap is set to about 1 mm. In this case, the nozzle 12 is turned manually, to adjust the length of protrusion of the nozzle 12.
In a laser beam machining operation, generally the laser beam 8 is focused on the upper surface of the workpiece 13; however, depending on the type of workpiece 13, it may provide a better result to focus the laser beam 8 on the lower surface of the workpiece 13.
In this case, the drive unit 11 is operated to move the inner holder 10a downwardly as much as the thickness of the workpiece 13.
The conventional laser beam machining apparatus is constructed as described above. Therefore, when the optical system (or lens) is moved vertically, the optical axis of the laser beam passed through the optical system may be affected because of the play of the drive mechanism. That is, the central axis of the optical system is parallel-moved or inclined relative to the optical axis of the incident laser beam. In order to overcome this difficulty, it is necessary to use the optical system holding mechanism and the optical system moving mechanism which are high in precision. Employment of such mechanisms will unavoidably increase the manufacturing cost of the machining apparatus. In addition, it is necessary to move both the optical system and its holder, and the movable components are considerably large in weight, and their drive units are also necessary bulky. Accordingly, the machining head is large as a whole, and its inertia is great. Thus, the response of the machining head to the control signal is lowered.
In the case where it is required to focus the laser beam on the lower surface of the workpiece, in order to maintain the gap best it is necessary to adjust the length of protrusion of the nozzle whenever the thickness of the workpiece changes, which requires lot of time and labor. Even if the laser beam should be focused on the lower surface of the workpiece during machining (cutting), sometimes it is preferable to focus the laser beam on the upper surface of the workpiece in forming a machining (cutting) start hole in the workpiece. In such a case, it is impossible to obtain the optimum gap for both the machining operation and the machining-start-hole forming operation.